It is not a choice of either/or. 

It is both/and.

A mature believer is committed to the entire Great Commission of Jesus in Matthew 28:19-20. They are being discipled to “obey everything Jesus commanded.”

And part of that obedience is going out to make more disciples.

A person is not a mature Christian disciple until he reproduces more disciples.

We do not have a choice between developing disciples or reaching new believers. Christians are instructed to do both.